Some of the largest and most stunningly beautiful caves in the world are limestone caves. They are found in many different countries and include many of the longest caves ever discovered. Some of these structures contain stalactites and stalagmites, limestone cascades and exceptionally delicate crystalline features that sparkle under any light that is brought into the cave. Considering the large number of such underground tunnels and their wide distribution, it is natural that people might wonder how the limestone caves were originally formed.
Before a limestone speleothem, or cave formation, can exist, there must first be limestone. Limestone is a sedimentary deposit that contains high levels of calcium carbonate, usually well in excess of 50 percent. Calcium carbonate is a substance often found in the shells of many ocean creatures, including extremely small ones, but also those of corals and barnacles. It isn’t surprising, then, that many limestone rock beds were laid down millions of years ago in locations that were covered by shallow seas. Some limestone is surprisingly thick, laid down layer by layer and often later overlaid by other, usually harder, rock.
Alkalinity of limestone
Calcium carbonate has basic, or alkaline, properties. It is also a relatively soft substance. These two traits set the stage for the formation of a limestone cave, provided that slightly harder and more neutral rock overlays the limestone. Since acidic substances neutralize alkaline substances, all that’s needed for the cave to be made is something that is acidic and which has the ability to dissolve the calcium compounds. There is such an acidic source widely available: rainwater.
Acidity of rainwater
While there can be considerable variation in how acidic rainwater is, as the rain falls, the water often becomes weakly acidic by falling through the nitrogen, carbon dioxide and sulfur found in the atmosphere, dissolving some of these atmospheric chemicals. The accumulated rainwater then seeps down through cracks in overlaying rock until it reaches the limestone. Though the process tends to be slow, the acids in the water counter the alkalinity of the limestone while dissolving calcium compounds. The wearing away of part of the limestone eventually forms a cavity through which the water can flow. The more easily it can flow, the more limestone it may wear away, sometimes creating large underground rivers. A few of these have been found that can even be navigated.
The underground rivers and streams often break through to the surface to eventually join the streams and rivers people are more familiar with and that flow atop the ground. If the water level underground drops, or if the water wears down to another level of limestone, the vacated tunnel that is left behind is the limestone cave. The site of the outlet usually forms the mouth of the cave, though there can be multiple cave entrances. At times, collapses and sink holes can form nearly vertical openings to the cave structure. Surface water can also create the vertical shafts.
As long as there is limestone and acidic water, the limestone continues to be sculpted by nature. Dissolved calcite is deposited, as well, as drops of calcium-rich water drip from the cave ceiling or flowing down the walls of the cave. This is what forms the stalactites and stalagmites or other formations. The cave structure can and usually does continue to grow gradually. The cave might one day collapse; however, until that happens, the caves continue to exist while constantly changing in very small and normally slow fashion.
Limestone caves occur in an enormous number of countries. They can be found in Europe, Asia, North and South America, New Zealand, the Philippines and many more places. Carlsbad Caverns National Park in the United States is only one of hundreds of examples of limestone caves. Sometimes the caves are fairly small. Sometimes they can stretch for many miles and some of the chambers can be extremely large. Almost every year, new caves are discovered, and the exploration of known caves also continues.
Although this is over-simplifying it in the extreme, one could say that limestone caves are primarily made by the interaction between long-dead marine life and rainwater or surface water. The existing caves continue to change and new ones continue to be formed in a simple process that can take tens of thousands or millions of years. In a way, then, stepping into one of these exceptionally beautiful nature-made structures is taking a step into the distant past. Spelunkers, when they explore these caves, become time travelers of a sort.